1. Field of the Invention
The present invention relates generally to the field of condensate drain equipment for steam header/piping systems and, more particularly, to a system and method for automatically draining condensate from steam sootblower header/piping systems for fossil-fueled steam generators used in electric power generation.
A fossil-fueled steam generator utilizes the stored chemical energy contained within the remains of fossil vegetation as the source of heat. Combustion of the fossil fuel releases this stored chemical energy which, in turn, is used to heat water and generate steam. By expanding the steam through a turbine connected to a generator, the energy in the steam is converted to electricity.
The combustion process generates hot combustion gases and, in most instances, residues known as "ash". The transfer of heat to the water/steam is accomplished by passing the hot combustion gases across banks of tubes known as heating surface, and through which the water/steam flows. The heating surface can be water cooled, superheater or reheater surface, depending on the fluid flowing therethrough. Continuous operation of the steam generator causes ash deposits or soot to build up on the tubes, decreasing the heat transfer efficiency. The deposits must be removed to restore the thermal efficiency and this removal is accomplished through the use of sootblowers. A sootblower directs a jet of high pressure media against the tube surface to dislodge the accumulated ash deposits and clean the heating surface. The sootblowers generally employ saturated or superheated steam as the blowing media. The sootblower steam source is generally the steam generator itself, and thus the operation of the heat transfer process in the steam generator has a direct effect upon the temperature and pressure of the steam for the sootblowers.
As would be expected, various areas of the steam generator's heat transfer surfaces have different cleaning requirements. The type of deposit and heating surface being cleaned determines the frequency and duration of a sootblower cleaning cycle, as well as the performance requirements of the sootblowing steam. Due to the nature and size of the steam generators themselves, and their arrangement of heat transfer surfaces located in the path of the combustion gases, elaborate steam header/piping systems are required to transport the steam from a given source in the steam generator (for example, a header connected to a bank of superheater or reheater heat transfer surface) to a given sootblower, while still providing the proper steam temperature and pressure to the sootblower to adequately clean the tubes during a cleaning cycle.
2. Description of the Related Art
The aforementioned steam header/piping systems require some means of warm-up to ensure that the proper degree of superheat is maintained and to rid the system of condensate before initiating a sootblowing cycle. This is usually accomplished by blowdown, using steam to purge and heat the sootblower piping. In the past, various methods and apparatus have been used to accomplish blowdown. Very early approaches were simple, manually operated, orificed drain valves. Later approaches incorporated automatic float and thermally operated traps. The present state of the art also includes thermally controlled, air operated drain valves; i.e., the thermal drain. FIGS. 1A, 1B and 1C show the aforementioned methods in their simplest form. FIG. 1A shows the use of the manual orifice, while FIG. 1B shows the use of a steam trap. Both of these approaches can be used where the sootblowing media is either saturated steam or superheated steam. FIG. 1C shows the use of the thermal drain to eliminate condensate from a piping system. Generally the thermal drain has a HI and LO temperature setpoint, at which the valve closes or opens, respectively, with a 50.degree. F. temperature differential therebetween. The thermal drain is thus generally used only with superheated, and not saturated steam, because the difference in temperature between hot condensate and saturated steam at a given pressure is practically nil. All of these blowdown methods are still in common use today. The thermal drain and trap have both evolved into sophisticated complex units in an attempt to improve operating reliability.
As used herein, the terms "saturated steam" and "superheated steam" are used in their ordinary thermodynamic context as known to those skilled in the art. The term "saturation temperature" designates the temperature at which vaporization takes place at a given pressure, and this pressure is called the "saturation pressure" for the given temperature. Thus, for water at 212.degree. F., the saturation pressure is 14.7 psia, and for water at 14.7 psia the saturation temperature is 212.degree. F. If a substance exists as a liquid at the saturation temperature and pressure, it is called saturated liquid. If a substance exists as a vapor at the saturation temperature, it is called saturated vapor, and if water is the substance, it is called "saturated steam". Similarly, when the vapor is at a temperature greater than the saturation temperature, it is said to exist as superheated vapor, and if water is the substance, it is called "superheated steam". For further details, the reader is referred to Fundamentals of Classical Thermodynamics, Second Edition, Van Wylen and Sonntag, John Wiley and Sons, Inc., 1973.
There are problems with the use of traps and thermal drains. While the steam generators have a full load maximum steaming capacity in pounds per hour of steam at a required temperature and pressure, the steam generator often operates at lower loads. The rate of heat transfer between the combustion gases and the water/steam flowing through the heat transfer surfaces changes with boiler load. A point which produced steam at a certain temperature and pressure at maximum load will produce steam at a different temperature and pressure at lower loads, and yet the sootblowing steam media requirements to clean a bank of heating surface remain the same. Variations in fuel quality can also change the combustion process and heat transfer distribution that results, as well as affecting the cleanliness of individual banks of heat transfer surface within the steam generator. Further, seasonal ambient variations in temperature can also affect the steam generator's performance. As mentioned earlier, thermal drains will not work with saturated steam. Steam traps can easily stick open and/or closed due to the low actuating force derived from the controlled media.
FIG. 2 is a schematic of a typical prior art system. As shown, this system uses a single thermal sensor located near the drain, such as a thermal well or thermostatic trap, to provide a signal that is compared to a manually adjustable setpoint for drain valve control. In such systems, the drain valve is opened to drain condensate when the steam temperature (T1) is less than the setpoint temperature (T.sub.MIN). The drain valve is closed when T1 is greater than or equal to the minimum allowable normal operating temperature (T.sub.NORM). T.sub.NORM is defined as: EQU T.sub.NORM =T.sub.MIN +T.sub.DIFF
where T.sub.DIFF is a margin or setpoint differential added to the setpoint temperature, T.sub.MIN. Between T.sub.NORM and T.sub.MIN is a "deadband" where the drain valve will retain its position (open or closed) until T.sub.NORM or T.sub.MIN is reached.
In some cases, however, the source steam temperature (T.sub.SOURCE) varies more than the setpoint differential, T.sub.DIFF. This condition renders the thermal controller useless until it is readjusted, leaving the drain valve continually open (dumping steam) or continuously closed (resulting in loss of superheat and/or condensate buildup). Changes in ambient temperature can also cause thermal sensor response errors greater than T.sub.DIFF, leading to the same problem.
Accordingly, it has become desirable to develop an improved condensate drain controller which overcomes the problems of the prior art.